Power hydraulic brake device and system



July 17, 1962 R. E. SCHWARTZ ET AL 3,044,265

PowEE HYDRAULIC BRAKE DEVICE AND SYSTEM Original Filed March 28, 1955 United States Patent O 3,044,265 POWER HYDRAULIC BRAKE DEVICE AND SYSTEM Robert E. Schwartz, Clayton, Mo., Roy P. Stahl, South Bend, Ind., and Edward J. Falk, St. Louis, Mo., assignors to Wagner Electric Corporation, St. Louis, Mo., a corporation of Delaware Continuation of abandoned application Ser. No. 497,272, Mar. 28, 1955. This application Mar. 15, 1960, Ser.

Claims. (Cl. 60-52) This invention relates to power hydraulic systems and the controlling means therefor, and in particular, toapower brake system and servo motor utilizing separate power hydraulic pressure uid and brake iiuid.

In general, this invention comprises an operator controlled servo motor serially arranged with positive displacement pumping means in a power hydraulic system v .deterioration and dilution of one pressure fluid by the other when attempts have been made to use separate power and actuation fluids. However, because of the improved iluid operating characteristics produced by using separate power hydraulic pressure fluid and brake iluid, it is advantageous and desirable to provide an improved servo motor having isolated power and brake actuating portions.

One of the principal objects of the present invention is to provide a servo motor which completely separates the lluid in the series power hydraulic portion of the system from the lluid in the braking portion of the system.

Another object of the present invention is to provide a braking system operated by a servo motor in series with a power system, but which functions independently of the ambient pressure in any section of the power system.

Still another object of the present invention is to provide a servo motor which provides feel yfor the operator. Feel is the proportional reactionary force in opposition to the applied force aording the operator an appraisal as to the extent of application.

Another object of the present invention is to provide a servo motor which operates even in the event of power failure to provide safe operation of the braking system.

A still further object is to provide 1a servo motor having novel pressure liuid compensation means for a brake cylinder portion thereof.

The ,invention also consists in the parts and in the arrangements and combinations of parts hereinafter described and claimed. AIn the accompanying drawings which form part of this specification and wherein like numerals and symbols refer to like parts wherever they occur:

FIG. 1 is a diagrammatic view of a power hydraulic braking system with a longitudinal cross-sectional view of the power hydraulic brake device shown therein, and

FIG. 2 is a fragmentary cross-sectional view showing the compensation valve of the preferred embodiment in detail.V

This is a continuation of copending application, Serial No. 497,272 tiled March 28, 1955, now abandoned, for

Power Hydraulic Brake Device and System.

Referring to FIG. 1, the power hydraulic system comprises a pumping means 1 having one side connected to a ice 7 arranged in series with said servo motor 4. The outlet of they steering valve 7 is connected to the reservoir 2 by a conduit 8 which provides a return passage for the oil displaced by the pumping means 1. A conduit 9 having a pressure relief valve 10 interposed therein connects the conduits 3 and 5, the relief valve 10 protecting the pumping means 1 from pressure overload above a predetermined value. A conduit 11 connects the conduits 6 and 8 and has a reverse dow valve 12 interposed therein to facilitate pressure fluid recirculation during a power failure steering application. If desired, the valve 12 may be integral with the power steering valve 7. The braking system includes a plurality of wheel cylinders 13 connected to the brake actuating portion ofthe servo motor by a conduit 14. A conduit 15 connects a brake lluid reservoir 16 to the servo motor for compensation purposes. According to the present invention, the power system contains a mineral oil base pressure Huid and the braking system contains a typicalhydraulic brake fluid.

The servo motor 4 comprises a booster housing 17 having a brake portion housing 18 secured to the right-hand end and dening a pressure producing outlet chamber or master cylinder chamber C, and an intake housing 19 secured to the left-hand end of the booster housing 17 and defining therewith a power portion of the servo motor 4. Th power portion includes an inlet chamber A and an outlet or return chamber B as will be more fully dened hereinafter. The intake housing 19 has a rod receiving bore 20 with a sealing assembly 21 supported therein and an axially aligned counterbore 22. The counterbore 22 receives a leftward extension of the booster housing 17 which has a counterbore 23 therein the extension supporting an O ring 22 which seals between the booster housing 17 and the counterbore 22. The intake housing 19 also has a vertically disposed inlet 24 to which the conduit scribes a guide extension 31 of the valve 26. The right end of the spring 30 abuts against a compression adjusting spacer 32 interposed between the spring 30 and the right end of the counterbore 28. An 0 ring 27, provides a seal between the housings 17 and 19 circumscribing the bore 27 and the counterbore 28. The pressure relief valve 26 is provided in by-pass relation withthe power chambers A and B and is biased by the predetermined force of the spring 30 into normally sealed position with the seat 29. However, power hydraulic pressures developed in the inlet chamber A by power control means including a piston rod 33 and a throttling valve 39 for actuating a power piston 41 to develop braking pressures in the pressure producing chamber C may act on the effective area of the valve 26 to overcome the force of spring 30 in order to prevent overloading the pumping means 1. p

The piston rod 33 includes a/leftward portion 33 slidably mounted in the bore 20 and receiving a linkage 34 pivotally mounted between the rod 33 and one end of a push rod 35. The other end of the push rod 35 is operatively connected with suitable actuating means such as an actuating pedal 36 having intermediate linkage (not shown) interposed therebetween. A resilient boot 37 ing 19 prevents the entry of foreign materials, but the effective area of the portionV 33 to the left of the seal Z1 4is subjected. to atmospheric pressure.. j The rod 33 extends coaxially into a counter-bore 38 of va leftward sleeve portion38"V ofthe throttling valve 39 andis spaced'therefromby a plurality of ears 4Q to pro-Y vide an f annular' passage 40'V in communication lwith the inlet chamber A. The right end .of the rod 33 issecured to the valve 39,. The power piston 41 is slidably positioned in the counterbore 23 and divides the power chamber into the inlet and outlet chambers .A vand B. The piston 41 includes an inner-bore 42 an intermediate .bore

the outer 'bore44 and has a radially-extending annular ange 45 therein'and the valve 39 also includes a right- Ward portion 39' extending intoY the intermediate bore 43 Y 43, and an outer bore .44. The valve 39 is positioned in ofthe power piston 41.V A plurality of angular-ly arranged passages 46 are formed in the valve 39 connecting the counterbore 3S with the outer bore 44, and the outer circumferential vportion at the left end of piston 41 having aplurality of throttling passages 47 providing communication between the outer bore 44 and the charnber B.V rI'hepassages 47 form a throttling passage D in combination with the radial ange 45 kof the throttling valve 39,

A seal assembly 48 is provided in the outer bore 44 between .the bore .44.and theperiphery of the rightward l portion 39' of the valve 39. The rightward portionv 39 .and the leftwardtportion 33. of the 'piston 33 have the same diameters yand thereforeA have the'same eiective end Y areasp outwardly. of the seals 43 and 2,1, respectively. In

order to provide a balanced power piston' arrangement as taught herein, it is necessary to subject theseV elective areas to the same'surrounding or ambient pressure which in this case is atmospheric. A spring 49 of negligible force'isV mounted in the ,inner bore 42 biasing the radial vpiston 41 to inoperative or retracted position.

flange V4,5 'of the valve 39 leftwardly to its open or unrestricted position abutting a stop. ring 5u supported in 'the outer bore 44. It 'will be. noted ,that the stop member 56 is positioned intermediate the axial extent of vthrottling passages'47, and that the radial ange 45 is movable with the throttling valve 39 to a throttlingr'position restricting but not closing the passage D. In both. positions, the leftward side of the radial' ange 45 is subjected to the same fluid pressure that prevails in return chamber'B vand the rightward side of the liange.45 is V.subjected to charnberA fluid pressures. .The opposed electiveareas of the piston rrod 33 and throttling valve 39 between the seals 21 and 48 areequal. j

The valverl39 carries aA seal assembly 51Vwhich extends into sealed condition with the/counterbore 2,3V at the left 4end Vof the piston 41 andseals betweenA the chambers A and B, the seal assembly-51 lhaving a retainer member to keep the sealing cup in sealing positipn. The retainer acts as. as top member for the piston rod 33 and throttling valver3/9U against the left end of the intake bore `39of the throttling valve 39 is subjected to atmospheric pressure to odset or balance the equal and opposite force on the leftward end of the portion 33 of the piston rod 33. It is also apparent that atmospheric pressure acts on the effective area of the power piston 41 in the bore 42 and this area is balanced by an equal. eiective area of the piston 41 in the separation chamber 53 outwardly vof the seal 52.

The Abrake housing 18 partially extends into the counterbore 53 having a bore 57 which defines the outlet chamber C at the right end thereof, and which slidably receives a master cylinder portion 58 which is retained therein by suitable means; The piston 58 has a leftward sleeve or annular projection 59 with a long recess or bore 6? to receive an extension 61 of the power piston 41. The extension 6'1 and sleeve 6 0 have an axial extent of suicient length to assure rectilinear actuation of the piston 58 in the chamber C. It is apparent that the pistons 41 and 58 may be integral so that the power piston has an extension with a piston working end in the outlet chamber C. A resilent boot 62 is interposed between the sleeve 59 and the brake housing 18 in ci-rcumscribing relation therewith and prevents the entry of foreign matter. An annular spacer 63: is positioned against .the right end wall SG of the counterbore 53, and a spring 64 extends from the spacer against a retainer 65 carried by the extension 61 and interposed in a step 79 formed between adjacent vertical walls of the pistons 41 and S8. The spring 64 is of negligible force only great enough to bias the retainer 65 against the left end wall 81 of the separation chamber 53'adjacent to the power portion ofthe servo motor 4 for returning the power The retainer 65 may function as a deector or drip guard to prevent llow of any power hydraulicl liuid seeping past seal 52 from migrating along thepiston sleeve 59.

A primaryrseal 66 is provided in chamber C for the working end Vof the piston 5S and a spring and retainer assembly 67 biases the seal 66 into an abutting relationship with the piston 5S. The sleeve 59 also carries a secondary seal 68 in sealing contact with the bore 57, the seal68 being formed of a material toseal brake luid. It is apparent .from FIG. l that the separation chamber 53 has an axial extent greater than the maximum stroke of the pistons 41 and 58, and that the piston surfaces sealedby sealing assemblies 52 and 68 are neverV in contact with other surfaces whereby transfer of power and brake uids Vis prevented.

A reservoir port 69 is vertically disposed in the brake housing 18 for compensation purposes. One end of the port 69 receives the conduit'15 and the other end 69 intersects the bore 57 adjacent to the lip of the seal 66 22, and is also perforated for the `free. passage of power r;

hydraulic fluid'. The. lopppsite, eiectivevareas of the power piston 41 in chambers A and B and across the sealvins vcui 5.1-areequa1... Another Sealing assembly Slis positioned inthe` rightiend of the counterbore 213 and provides a seal raround the piston 41, `the seal, 5 2 and f. y

c ah of the other seals in the powerportion of the servol motor being Oia .material adapted mineralV oilA base power iiuid,

forv long life with A uid separation counterbore or chamber` axially passage 54, the passage being an escape for any pressure in its normal inoperative position. in FIG. 2, a compensation valve 76 is biased by a spring 71 into a seat *'72 formed-by the juncture of a bore 73 and an axially aligned counterbore 74 horizontally disposed in the brake housingd, the right end of said spring 71 abutting against an end plug 75 iixedly received in the counterbore 74. The left Vend of the bore 73 is connected to the lreservoir port 69 by a cross-drilled passageway 76. Another cross-drilled passage 77 isprovided in the brake housing 18, one end thereof intersecting the counterbore 74, the midfportion-thereof being in communication with the bore 57,.and the otherend.formingabrake port 7S.

- The brake port 78 receives the conduit 14, FIG. 1, conaligned with the counretbore 23 andj is in communication :e

-with the inner bore 42 of the piston 41 by means of .a

fluid which ymight leak pastY the sealing assembly 48. A

passage 55 having an air lter 56 is vertically disposedpin the booster housing V17 injcommunication withthe sepav ration chamber 53 at Vthe left end thereof and provides a drain for any uids which might collect in the separation necting the bore 57 Vwith the brake assemblies.

`In vthis series system, the pressure relief valve 10 actuates at a pressure equal to the sum of the maximum pressure' differentials allowed .across the-individual servo motors'thereb-y protectingthe pumping means 1 from pressure overloads. Pressure overloading of the pumping means 1 developswhen the servo motor 4, or other servo Vmotors 'of other devices such as steering valve 7, exceeds the predetermined maximum pressure difierennew" tial assigned thereto. The servo motor 4 is provided with a pressure relief valve 26 which limits the magnitude of the maximum pressure differential allowed thereacross. However, the fluid pressure existing in a servo motor at any time is effective upon every servo motor of prior position in the system. For instance, if the steering valve 7 is actuated, the pressure differential created thereacross causes a back pressure which is effective throughout the servo motor 4 and against which the pumping means 1 must work. Ordinarily, a servo motor of prior position would be actuated by this back pressure. However, the servo motor 4 is balanced to prevent such an occurrence, this balancing being achieved by a symmetry of cross-sectional areas which are affected by the aforesaid back pressure. The opposing effective cross-sectional areas of the piston 41, the valve 39, and the valve rod 33 respectively are constructed to be equal and self-cancelling when acted upon by the ambient iiuid pressure existing in that particular section of the hydraulic system. Atmospheric pressure is also prevalent acting upon opposing effective cross-sectional areas of the piston 41, and the valve 39 and the valve rod 33. However, the forces thus created are also equal and self-cancelling. Consequently,.regardless of the ambient pressure the servo motor 4 is balanced to function independently in the series system.

. Assuming the steering valve to be unrestricted,\pres sure uid is normally discharged from the pumping means 1 through the conduit 5 and the inlet 24 into the inlet chamber A of the servo motor 4. The pressure uid iiows between the valve rod 33 and the valve 39 through passage 46 and through the passages 46, the outer bore 44, and the normally unrestricted throttling passages 47 into the outiet chamber B. From the outlet chamber B the uid flows by means of the outlet 25 and the conduit 6 into the power steering valve 7 and is returned to the reservoir 2 through the conduit 8.

When the vehicle operator applies a force to the actuating pedal 36, a throttling force is transmitted to the push rod 35, the linkage -34, the valve rod 33 and the throttling valve 39 which is moved rightwardly restricting pressure fluid ow through the throttling channel D between the flange 45 and the right end of the throttling passages 47. Consequently, a pressure differential is created across the power piston 41 between the chamber B and the portion of the outer bore 44 of the piston 41 between the piston and the throttling valve flange 45 to develop a back pressure against which the pumping means 1 must work. The pressure differential also exists across the sealing cup 51 .between the chamber A and -5 the portion or the bore 44 between the seal 51 and the left side of the throttling valve flange 4S. In effect, the pressure differential acrossthe piston 41 exists in the chambers A and B, and this pressure differential is also exerted as a reactionary force on the throttling valve ange 45 in opposition to the actuating force thereby providing feeL Because of the existing pressure differential between chambers A and B, the piston 41 moves rightwardly` compressing the spring 64 in the separating counterbore 53 and actuating the piston 58 and the seal 66 further into the outlet chamber C to develop brake pressures in the wheel cylinders 13. The valve 39 and valve rod 33 are maintained in a restricted, but not closed, position relative to the piston 41 by the manually applied force in opposition to the pressure of the power fluid acting on the rightward side of the radial flange 45 of the throttling valve 39 in order to continue the braking application. This movement displaces brake Huid from the pressure producing chamber C through the brake port 78 and the conduit 14 into the wheel cylinders 13 thereby actuating the brake assemblies. The brake fluid pressure developed in the chamber C acts on the effective area of the seal 66 creating a reactionary force in opposition to the motivating force of the piston 41. When the reactionary force equals the motivating force,

the rightward movement of said piston 41 ceases. In the above described case, the operator is afforded a feel as to the extent of the braking application in that he feels the created pressure differential on the effective areas of the valve rod 33 and the 'valve 39 which is directly proportional to the hydraulic output to the wheel cylinders 13.

When the operator releases the applied force, the established uid pressure in the Ibore 57, the spring 64, and the spring 67 returnsthe piston 41, the piston 58, and parts associated therewith to their normal positions. When this occurs, a partial vacuum is developed inthe bore 57, and the pressure differential across the valve 70 opens the valve allowing brake fluid to compensate into the bore 57. Therefore, the vacuum created in the bore 57 is overcome and any excess uid present when thev wheel cylinders are in inoperative position is returned to the reservoir line 15 through port 69'.

In the event of power failure, the operator is required to supply all the actuating force in operating the servo motor 4. As previously described, the force applied by the operator motivatesthe valve 37 rightwardly. Due to the absence of pressure fluid circulation through the throttling passages 47, restriction of -the passage D does not develop a pressure dilferential -to motivate the power piston 41. Consequently, the right end of the valve 39 abuts against the power piston 41 and carries -the piston 41 and the piston 58 rightwardly. Upon this rightward movement, the lip of the sealing cup in the sealing assembly 51 will collapse allowing the pressure uid .to ilow thereby. Therefore, the use of a reverse flow valve allowing pressure fluid recirculation is obviated. This rightward movement also displaces brake fluid from the outlet chamber C through the :brake port 78 and the conduit 14 into the wheel cylinder assemblies 13 to actuate 4the brake assemblies. Upon release of :the applied force, the brake fluid pressure in the lbore 57 and the compressive force of the springs 64, 67 and 49 return -the power piston 41, the piston 58, the valve rod 33 and ports associated therewith to their normal position.

The construction of the servo motor 4 completely isolates the oil based pressure iluid used in the power hydraulic system yfrom the brake uid employed in the braking system. Any oil based pressure fluid leaking past lthe `sealing assembly 48 in the power piston 41 will flow through the passage 54 into the separation cham-ber 53,

f and any pressure fluid leaking past the sealing assembly 52 flows directly linto the chamber S3. Therefore, any

,pressure fluid collecting in the separating ychamber 53 will drain therefrom through the passage 55. The braking system is protected fromthe/entry of pressure uid ltherei-nto Iby the 'boot 62 in the `chamber 53.` If any brake ud escapes past the secondary seal 68, it will collect in the boot 62. However, if said boot 62 should -rupture, the brake fluid will drain from the chamber 53 through the passage 55 and in any event no transfer of iiuids will be effected in continuous operation of the servo motor wi-thout a boot 62 since yno seal for one fluid wipes a surface exposed to lthe other fluid. Therefore, the servo motor 4 completely separates the pressure fluid in the power hydraulic system from the brake liuid of the braking system.

This invention is intended to cover all changes and modifications of the example of the invention herein chosen for purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention.

What we claim is:

1. A power hydraulic servo motor comprising a housing having a -bore with a piston assembly slidablel therein, a varla'ble orifice in said piston assemibly through which a first pressure fluid is continuously circulated, an outlet chamber containing a second pressuer fluid, an extension for said piston assembly extending into said outlet charnber, -said bore and outlet chamber being spaced apart by a separation chamber through which extends said piston 7 assembly extension, means fork restricting said variable orifice to `throttle -the `continuous flow of said rst pressure uid -to move said extension in said separation and outlet chambers, sealing means for preventing leakage of first and second pressure fluids into said separation chamber, and means in said separation chamber to prevent cornmingling of the first and second pressure fluids along said extension. V Y

2. In a device of the 'type described including Yfirst and second chambers and a reoiprocable member in one of said chambers with a portion extending 'into the other of said chambers for reciprocating movement therein, said first and second chambers containing first and second fluids of an incompatible nature, the improvement comprising a separation cham-ber 'intermediate said first and valve movable relative to said piston member for developing a differential in the magnitude of pressures of said rst pressure fluid acting on opposite sides of said piston member, said piston assembly being movable a predetermined maximum distance inY response to the pressure differential in said power chamber and having a pistonl working end slidably positioned in said pressure producing chamber; lthe improvement which comprises a separation chamber in said housing spacing said power and pressure producing chamber yapart a distance greater than the maximum movement of said piston member, -said piston assembly having a first extension portion extending from said power chamber into said separation chamber, iirst pressure uid sealing means between said power and separation chambers slidably receiving said'iirst extension portion therethrough, said piston assembly havingk a second extension portion of greater diameter than said first extension portion and extending from adjacent to. said first sealing means into said pressure producing cham- Vber and having said piston working end formed thereon,

second pressure fluid sealing means for said second extension portion between said second and separation chambers, and means intermediate said extension portions of said piston assembly for stopping `axial leakage ow of first and second pressure uid along said rst and second extension portions, respectively, Vand onto the second and first extension portions, respectively, whereby commingling of fluids on said piston assembly is prevented.

4. In a hydraulic servo motor including a housing having a power chamber containing a first pressure uid and pressure producing chamber containing a second pressure fluid, said power chamber including `a power piston and valve means movable relative to said power piston for developing a differential in the magnitude of pressures of' said first pressure fluid acting on opposite sides of said power piston, said power piston being movable a predetermined maximum distance in response `to the pressure diferential in said power chamber and having piston exten'- sion means forming -a working end positioned in said pressure producing chamber; ,the improvement which comprises la separation chamber in said housing spacing i said power and pressure producing chambers 'apart a distance greater than Ithe maximum movementof said power piston, first sealing means for said piston extension means between said powerr land separation chambers, secondV sealing means for lsaid piston extension means between said pressure producing and separation chambers, saidV piston extension means having stepped circumferentialv surfaces adjacent to said fir-st seal interrupting axial ow of first pressure fluid leakage -along said piston extension means,V and la yieldable accordion seal housing said piston extension means in said separation chamber having one end secured to said piston extension means adjacent to said stepped surface and the other end securedtto the wall of said separation chamber adjacent lto said second seal in `circumseribing relation therewith.

Y 5. A hydraulic servo motor comprising a housing having a bore and a pressure producing outlet chamber spaced apart by a separation chamber vented to atmosphere, a piston slidable in said bore dividing it into first and second chambers and an extension on said piston extending through said second and separation chambers and having a working end in said outlet chamber, said piston having a central chamber therein in communication with said separation chamber and a throttling passage formed through said piston in communication with said first and second chambers, pumping means continuously circulating pressure uid through said throttling passage and first and second chambers, a throttling valve in said first chamber having one end slidable in coaxial arrangement in the central chamber of said piston and the other end positioned externally of the housing, said throttling valve including an annular radial flange positioned in said throttling passage, said throttling valve being movable relative to said piston by an applied force for reducing the size of said throttling passage for developing a pressure differential between said first and second chambers actuating said piston to move the working end of said extension in said outlet chamber, said developed pressureV differential also acting on said annular flange of said throttling valve in opposition to the applied force on said throttling valve.

6. A hydraulic servo motor in an open center fluid pressure system, said servo motor comprising a housing having a bore and a pressure producing outlet chamber spaced apart by a separation chamber vented to atmosphere, a piston slidable in said bore dividing it into first and second chambers and an extension on said piston extending through said second and separation chambers and having a working end in said outlet chamber, said piston having a central chamber therein vented to atmosphere through said separation chamber and a throttling passage formed in said piston in communication with said first and second chambers, pumping means continuously circulating pressure fluid through said throttling passage and rst and second chambers, a throttling valve in said first chamber having one end slidable in coaxial arrangement in the central chamber of said piston and the other end positioned externally of said housing, said throttling valve including an annular radial flange positioned in said throttling passage and movable between inoperative and operative positions, spring means in said central chamber normally biasing said throttling valve to the inoperative position in which said throttling passage in said piston is unrestricted, an annular seal between said throttling valve and piston intermediate said throttling passage and said central chamber, said throttling valve being movable to operative position relative to said piston by an applied force for reducing the size of said tbrottling passage to develop a pressure differential between said rst and second chambers actuating said piston to move the working end of said piston extension in said outlet chamber, and said developed presf sure differential also acting on said `annular flange of said throttlingV valve in opposition to the applied force on said throttling valve.

7. A hydraulic servo motor in an open center fluid pressure system said servo motor comprising a housing having a bore a separation chamber,l anda pressure producing outlet chamber aligned therein, pumping means providing i a substantially continuous now of first pressure fluid in the system, an inlet and an outlet serially connecting said bore with said pumping means, a source of second pressure fluid connected to said outlet chamber, a drain port connesting said separation chamber with the atmosphere, a

piston assembly slidable in said housing bore between said inlet and outlet and having a piston extension extending through said separation chamber and having an end portion slidably disposed within said outlet chamber, a bore in said piston in communication with said separation chamber, a throttling valve member having opposite and substantially equal elective end areas one of which is slidable in said piston bore and the other being positioned externally of said housing and subjected to atmosphere, said throttling valve member providing a throttling passage in combination with said piston assembly to accommodate the continuous flow of irst pressure fluid between said inlet and outlet in said housing bore, the eective areas of said piston assembly and throttling valve member in said housing bore on the outlet side thereof being substantially equal to the efective areas of said piston assembly and throttling valve member in said housing bore on the inlet side thereof to prevent movement of said piston assembly and throttling valve member in response to variations in the magnitude of pressure of the rst pressure fluid in the system acting on the opposed effective areas of the piston assembly and throttling valve member, said throttling valve member being movable relative to said piston assembly in response to an external force to restrict the size of said throttling passage, said piston extension being movable to displace second pressure fluid from said outlet chamber in response to the throttling coaction between said throttling valve member and piston assembly, and means including said separation chamber to prevent commingling of the first and second pressure uids, said latter means comprising spaced seals for said extension between said separation chamber and bore and between said separation chamber and outlet chamber, respectively, and an expansible sealing member housing said extension in said separation chamber having one end connected to said extension adjacent to said bore and the other end connected to the end wall portion of said separa-tion chamber adjacent to said outlet chamber whereby the portion of said extension exposed to the pressure uid in contact with one of said seals never contacts the other of said seals.

8. A hydraulic servo motor for use in an open center uid pressure system, comprising a housing having a bore, a. separation chamber, and an outlet chamber aligned therein, pumping means providing a continuous oW of rst pressure fluid in the system, an inlet and an outlet for serially connecting said bore with said pumping means, an outlet port connecting said outlet chamber in a second pressure uid circuit, a drain port connecting said separation chamber with the atmosphere, a piston assembly slidable in said bore and having an extension extending through said separation chamber and having au end portion slidably disposed in said outlet chamber, a bore in said piston, a throttling valve member having one end extending externally of said housing and the other end slidable in said piston bore, the opposing eifective areas of said piston assembly, extension, and throttling valve member being in predetermined balanced proportion to prevent movement of said piston assembly, extension, and throttling valve member in response to variations in the magnitude of pressure of the lirst pressure uid in the system, a vent passage in said extension connecting said piston bore with the atmosphere in said separation chamber, said throttling valve member providing a throttling passage in combination With said piston assembly to accommodate the continuous flow of first pressure iiuid between said inlet and outlet in said bore, said throttling valve member being responsive to an external force to restrict the continuous flow through said throttling passage, said extension being movable to displace second pressure fluid fromsaid 70 outlet chamber in response to the throttling coaction between said throttling valve member and piston assembly, and means including said separation chamber to prevent commingling of the pressure iiuids of the system and circuit, said latter means comprising spaced seals for said extension between said separation chamber and bore and between said separation chamber `and outlet chamber, respectively, and an expansible sealing member housing said extension in said separation chamber having one end connected to said extension adjacent to said bore and the other end connected to the end wall portion of said separation chamber adjacent to said outlet chamber whereby the portion of said extension exposed to the pressure fluid in contact with one of said seals never contacts the other of said seals.

9. A hydraulic servo motor comprising a housing assembly having a first bore in one end thereof, a second bore in the other end thereof aligned with the irst bore, and a separation chamber in an intermediate pontion thereof isolating said iirst and second bores from each other, a piston assembly slidable in said first bore and having eX- tension means projecting through said separation chamber and having a piston member slidable in said second bore and defining a pressure producing outlet chamber therein, said piston assembly dividing said first bore into inlet and reservoir chambers on opposite sides thereof with said reservoir chamber being adjacent to said separation chamber, said piston assembly having a counterbore in opposed relation with said one end of the housing assembly, rod means projecting into said housing through said one end thereof in alignment with said counterbore, a throttling valve member at the inner end of said rod means slidably received in said counterbore, resilient means normally positioning'said throttling valve displaced yfrom a bottomed position within said counterbore, said throttling valve member and said piston assembly having passageway portions communicating said inlet and reservoir chambers, said Ihousing having an inlet leading to said inlet chamber and an outlet leading from said reservoir chamber for connection to a fluid-circulating system, and an outlet port leading from said outlet chamber for connection to a' separate slave system to actuate the same in Iresponse to movement of said extension means to force hydraulic Huid from the outlet chamber, said extension means having a passage therein leading from said counterbore to said separation chamber, said separation chamber having a drain vent therein, said throttle valve -member and said piston assembly having cooperable portions responsive to movement of said throttle valve member toward a bottomed position in said counterbore to restrict the passageway connection between said inlet and reservoir chambers, a seal engaging said extension means to seal said outlet chamber from said separation chamber, a second seal engaging said extension means between said reservoir chamber and said separation chamber, and a boot-like expansible sealing member engaged at one end around a portion of said housing assembly forming said second bore and `at its other end around said extension means.

10. The assembly as defined in claim 9 wherein said extension means includes a portion integral with said piston assembly and extending therefrom a substantial axial distance into said separation chamber, said extension means further including a second piston having a recess of substantial depth receiving the said piston extension and extending lfrom within said separation chamber into said outlet chamber to form said piston member, said expansible sealing member being engaged at said other end thereof with said second piston.

References Cited in the tile of this patent UNITED STATES PATENTS 1,890,010 Vickers Dec. 6, 1932 y1,967,893 Libbey July 24, 1934 2,844,941 Ayers July 29, 1958 

